The most critical part of your tire is the tread, which gives you the traction to stop and hold the road on curves. Tire tread also funnels water out from under the tire, which helps to reduce “hydroplaning,” where a car actually rides up on a layer of water and becomes dangerously difficult to steer or stop.
According to the NHTSA, about one out of every 10 cars on the road has at least one worn out or bald tire. There are several things you can do to stay out of that group, and help the tread last longer on your tires. For starters, make sure that your tire dealer balances your tires when installing them. Balancing involves placing small weights on the rim to counteract heavy spots, or slight variations in weight, in the wheel. If a tire is not balanced, it will shimmy as you drive, and your tread will wear down quickly. You must also make sure that your car’s suspension is properly aligned.
The quicker-and-dirtier way to test tire tread depth is to stick your finger in between the tread blocks. You should feel some little platforms in between the blocks, but sunk down well below the tread surface. The platforms are located at 2/32”, so when the tread surface becomes even with these “wear bars,” the tire needs to be replaced. If you can still feel some “meat” on the tread, all is not yet lost
If you want to test your tire tread depth the right way, you'll need to use a gauge. Available at just about any auto and tire store, tread depth gauges are inexpensive and they stow easily in a glove compartment. Most are a simple graduated mechanical probe, but there are fancy-schmancy digital models available as well. Stick the probe into the valley of the tread, press the shoulders of the probe flat against the tread bloke, and read off the result. All gauges should measure in both 32nds of an inch and in millimeters.
You should use a tire depth gauge to check tread depth every month or so, and check multiple spots as well. By doing this you can catch irregular wear, like that caused by alignment issues, long before it becomes an irrevocable problem.